Lithium ion batteries (hereinafter as “LIB”) are widely used in various kinds of electric apparatus, and also used as power energy of electric vehicles for its higher operating voltage, higher energy density, stable discharging curve, lower self-discharging, long life cycle, memoryless property and non-pollution.
The power energy has a much higher requirement than the small scale electric apparatus for LIB. Selection of an electrode material is a key factor that affects the performance of LIB. In addition to improve the material of the positive electrode, the electrolyte and the separator, the improvement of an anode active material is also necessary. The anode active material of the existing commercial LIB is mainly graphite, which has lower lithium insertion potential and excellent intercalation/deintercalation performance. Therefore, graphite is a good anode active material for LIB. The intercalation/deintercalation of lithium ions in graphite is calculated according to stoichiometry LiC6 and its theoretical capacity can reach up to 372 mAh/g. In general, the practical capacity of graphite is about 330 mAh/g, which is very close to its theoretical capacity, and it is difficult to further increase its practical capacity.
The lower capacity of carbon anode materials (such as graphite) restricts the energy density of LIB. Therefore, some non-carbon anode materials have attracted the attention of the industry because of higher energy density, wherein nickel oxide becomes new generation of anode materials of LIB for its high theoretical capacity (718 mAh/g), environmental friendly features, rich natural reserves and low cost.
As an anode material of LIB, volume change of the nickel oxide is large during the intercalation/deintercalation of lithium ions, which will easily cause the crush of the material. The poor conductivity performance of the nickel oxide leads to poor cycling stability and poor high-rate charging/discharging performance. On the other hand, the particle size and morphology of the nickel oxide also have an effect on its electrochemical performance. The structure of the nickel oxide and big particle size obtained by traditional manufacturing method causes poor conductivity performance in material and lower diffusion rate of lithium ions in nickel oxide particles, and these factors restrict the electrochemical performance of the nickel oxide and the cycling performance of the battery.
China application No. CN201310724013.9 discloses a composite anode material comprising a hollow porous nickel oxide coated by a nitrogen-doped carbon layer. Through surface modification of the hollow porous nickel oxide, the utilization rate and conductivity performance of the nickel oxide are improved. The LIB using this material as the anode active material has the characteristics of good discharging performance, high cyclic stability, and high capability.
China application No. CN201210546937.X discloses a coaxial composite anode material of carbon nanotube arrays with nickel oxide nanoparticles. The composite anode material comprises carbon nanotube arrays growing on a metal collector substrate in situ, and nickel oxide nanoparticles uniformly distributed on and directly combined with the outer surfaces of the carbon nanotube arrays, wherein the nickel oxide nanoparticles account for 50-85 percent by weight of the composite anode material. Thus, a composite anode material having high conductivity, good cycle performance, high specific capacity is obtained, and the composite anode material is not easy to aggregate and crush caused by volume change.
From the above, in order to control volume change of the nickel oxide during charging-discharging and to improve its conductivity performance, the prior art usually uses a good conductivity material such as carbon to coat the nickel oxide or combines the nickel oxide with the carbon nanotube arrays. Surface coating can solve the problems of crushing and volume changing of the nickel oxide particle during charging-discharging to a certain extent. However, because the active material of the nickel oxide is coated by a layer of carbon material, the lithium ions must pass through the coating layer firstly and then embed into the anode active material (i.e., the nickel oxide of the inner core) in the process of intercalation of lithium ions. Thus, the coating layer outside the nickel oxide will affect rate capability of the anode material as well as utilization of the inside nickel oxide. The carbon nanotube arrays growing on a metal collector substrate in situ and further combined with nickel oxide nanoparticles can be used to increase utilization of the nickel oxide, but this will raise the production cost.